Gas turbine engines, such as those found on jet aircraft, comprise a housing within which fuel and air are combined and burned. Such engines generally include vanes for drawing air into the housing and a compressor that compresses the air and sends it to a gas generator. In the gas generator, a precisely metered supply of fuel is mixed with the compressed air and burned. Expanding exhaust gasses turn a turbine which turns a shaft that provides power to the compressor, a fuel pump, and other elements associated with the engine. The exhaust gasses then pass through the remainder of the housing and exit the housing to provide thrust.
A starter, which is not part of the gas turbine engine or aircraft, is generally used to start a gas turbine engine. The starter causes the engine shaft to rotate at about 10 to 25 percent of its rated speed. This causes the compressor to compress air and the fuel pump to pump fuel to the gas generator. Once the fuel in the gas generator is successfully ignited, expanding exhaust gasses turn the turbine and the shaft and provide power for the compressor and fuel pump so that the starter is no longer needed.
Engines are generally designed to start reliably when their shaft is rotating at a certain percentage of rated speed. This does not mean that the engine will never start at a lower speed, but that starts at such lower speeds are not as likely to occur as when the engine is turning at an intended starting speed. It is generally desirable to turn an engine shaft at about 10 percent of its rated speed during engine start-up; at lower speeds starting may be difficult because, for example, the fuel pump may provide fuel at too low a rate for a reliable start.
Sometimes it becomes necessary to restart an engine while an aircraft is in flight. Air rushing through the engine housing of a non-functioning engine causes the blades on the compressor shaft to rotate and run the fuel pump. This rotation is sometimes referred to as “windmilling.” An engine restart while its compressor is windmilling may be referred to as a “windmill start.” The windmilling blades turn the engine shaft, but generally not at the 10 percent of rated engine speed considered desirable for a reliable engine start. In modern turbofan engines, for example, the windmilling blades may only turn the engine shaft at 5 percent of the rated engine speed. Other variables, such as airspeed when the restart becomes necessary, may further reduce the ability of the engine to be reliably windmill restarted.
The fuel pump used on a gas turbine engine must be large enough to provide fuel over a wide range of engine operating speeds. However, a fuel pump capable of supplying an engine with fuel under normal operating conditions may not supply adequate fuel for a windmill start. Therefore, it is known to provide gas turbine engines with fuel pumps that are, for example, 50 to 100 percent larger than necessary for normal engine operation to ensure that the fuel pump can provide an adequate fuel supply at low windmilling shaft speeds.
Using a fuel pump larger than is required for normal engine operation increases the weight and cost of a gas turbine engine. Furthermore, such a large fuel pump generates more heat than would be produced by a smaller fuel pump. This heat must be absorbed or dissipated by various heat sinks, such as the fuel supply, in the gas turbine engine, and this limits the amount of heat that these sinks can absorb from other sources. It would therefore be desirable to provide a gas turbine engine fuel system that would allow for reliable windmill engine restarts using a smaller fuel pump than has heretofore been possible.